STIMULATION AND TRANSMISSION 291 



interval also appears to be determined by the same 

 conditions. 



SUMMATION 



The phenomenon of summation is of great importance 

 in the analysis of the stimulation process. It shows 

 clearly that a single subminimal stimulus produces an 

 effect on the tissue, but that this effect is transient; 

 within a certain brief time the tissue resumes the same 

 condition as before the stimulus. But if before this 

 time has elapsed a second similar stimulus is applied, 

 its effect is added to that of the first, and the critical 

 level of disturbance required to initiate an excitation- 

 wave may be reached. The second stimulus, in order 

 to be effective, must be sent in before the effect of the 

 first has subsided; and the more rapid the rate of this 

 subsidence the shorter is the summation-interval. The 

 summation-interval is therefore defined as the longest 

 interval separating the successive subminimal stimuli 

 of an effective series of two or more such stimuli.^ 



This interval is shorter than the least duration of the 



exciting current of threshold intensity; and its precise 



duration varies with the intensity of the subminimal 



stimuli employed. Lucas gives the following intervals 



for different frog's tissues at 13°, using two subminimal 



electric stimuli (induction shocks) which were 5 per cent 



below the strength required for stimulation by single 



stimuli.^ 



Motor nerve (sciatic) 0004-.0005 sec. 



Muscle (sartorius) 0011-.0019 sec. 



Ventricle 008 sec. 



Cf. Lucas, Journal of Physiology, XXXIX (1910), 462. 

 Op. cit. (1910), pp. 466 £f. 



